Good adhesion properties, particularly to chalky substrates, are important to coatings such as paints and architectural coatings. One conventional method for improving the chalk adhesion properties of a coating composition is to add an alkyd resin to the conventional coating composition. However, alkyd resins produce premature yellowing, rendering the coatings undesirable in certain circumstances.
Another method for improving the chalk adhesion properties is to synthesize a polymer blend in a sequential polymerization. Examples of acrylic-based polymer blends made by sequential polymerization processes can be found in U.S. Pat. Nos. 5,990,228 and 6,710,112 B1.
The '228 patent discloses aqueous coating compositions containing at least two polymer components which supposedly provide adhesion and improved durability, as measured in dried coatings made from the aqueous compositions by improved gloss retention or dirt pickup resistance. Specifically, in Comparative Example 1 of the '228 patent, this patent discloses the use of a pre-polymerized latex core of 60 nm particulate poly(butyl acrylate-co-methyl methacrylate-co-methacrylic acid), upon which was further polymerized two layers of poly(butyl acrylate-co-methyl methacrylate-co-acrylic acid) of the same composition. The at least two polymer components are synthesized as core-shell polymers by a sequential polymerization process.
The '112 patent discloses aqueous polymer dispersions having two polymer phases that have different glass transition temperatures but that are formed by a sequential polymerization process. In addition, the molecular weights of the two polymer phases are different, due to the addition at some point during the sequential polymerization of a chain transfer reagent.
Other publications disclose alternatives to alkyd resin modification in coating compositions.
For example, U.S. Patent Application Publication No. 2004/0161542 A1 and U.S. Pat. No. 6,630,533 both disclose compositions containing at least one fatty acid ester. In the '542 publication, the fatty acid ester is unsaturated. In the '533 patent, the fatty acid ester comprises a C12 to C40 alkyl (meth)acrylate.
U.S. Pat. No. 5,376,704 discloses aqueous coating compositions containing a neutralized half-ester product of an acrylic polymer containing at least two reactive anhydride groups that is crosslinked with an epoxy crosslinker. The molecular weights of both these components are less than 100,000 Daltons.
U.S. Patent Application Publication No. 2004/0010091 A1 discloses two component coating compositions that cure under ambient conditions. The coating composition of the '091 publication contains crosslinkable and crosslinking components, with both components having molecular weights under 100,000 Daltons.
U.S. Patent Application Publication No. 2004/0010071 A1 discloses an aqueous polymer blend composition containing soft and hard polymer particles. The hard polymer particles have a glass transition temperature greater than 25° C., while the soft polymer particles have a glass transition temperature less than or equal to 25° C. In addition, the hard polymer particles constitute from 2-30 wt % of the composition, while the soft polymer particles constitute from 70-98 wt % of the composition.
U.S. Patent Application Publication No. 2005/0009954 A1 discloses aqueous polymer compositions containing hard polymers and soft polymers, with at least the hard polymers include phosphorus-containing and/or polyacid-containing pendant groups. The hard polymers have a glass transition temperature of at least 20° C., while the soft polymers have a glass transition temperature of from −20° C. to 5° C. In addition, the hard polymers constitute from 2-40 wt % of the composition, while the soft polymer particles constitute from 60-98 wt % of the composition.
In linear polymerization, there are several ways to measure average molecular weight, including weight average molecular weight (Mw), and number average molecular weight (Mn). The formulas for these average molecular weights are as follows:
                    M        _            w        =                            ∑          i                ⁢                              N            i                    ⁢                      M            i            2                                                ∑          i                ⁢                              N            i                    ⁢                      M            i                                and                              M          _                n            =                                    ∑            i                    ⁢                                    N              i                        ⁢                          M              i                                                            ∑            i                    ⁢                      N            i                                ,  
where Ni is the number of molecules of molecular weight Mi. Other average molecular weights include viscosity average molecular weight (Mv) and Z average molecular weight (Mz). During polymerization, the individual polymer chains do not have the same degree of polymerization, and therefore the weight of the individual molecules always has a distribution around an average value. See Textbook of Polymer Science, by Fred W. BillMeyer, Jr., (1984) at pp. 189 and 199.
The ratio of these two average molecular weights is the polydispersity index (PDI):PDI=Mw/Mn.Mw is more sensitive to the presence of high molecular weight molecules and Mn is more sensitive to molecules with lower molecular weight. Mw is always greater than Mn, except for a theoretical monodisperse system. Id. at p. 200. Hence PDI≧1.0.
PDI is an indication of the distribution of individual molecular masses in a polymer system. Higher PDI indicates higher variation of molecular masses and lower PDI indicates that the molecular masses are fairly uniform.
The latex polymer blend disclosed in the prior art reference US 2004/0010071 or U.S. Pat. No. 6,930,141 to Gebhart et al is dominated by soft or low Tg polymer particles (70%-99.7%) and has only a minor amount of hard or high Tg polymer particles. (See Abstract). The weight average molecular weight (Mw) of the soft polymer ranges from 75,000 to 2,000,000. Since the molecular weight of the soft polymer is high and the relative amount of the hard polymer is low, inter alia, the latex composition disclosed in Gebhart has poor adhesion to chalky substrate typically found outdoors.
There remains a need in the art for a polymer blend of soft and hard latex polymers that exhibits improved adhesion to chalky substrate.